Hydrothermal liquefaction of Chlorella vulgaris: Effect of reaction temperature and time on energy recovery and nutrient recovery

Hydrothermal liquefaction of Chlorella vulgaris feedstock containing 80% (w/w) water was conducted in a batch reactor as a function of temperature (300, 325 and 350 °C) and reaction times (5, 10 and 30 min). The biocrude yield, elemental composition and higher heating value obtained for various reaction conditions helped to predict the optimum conditions for maximizing energy recovery. To optimize the recovery of inorganic nutrients, we further investigated the effect of reaction conditions on the ammonium (NH₄ ⁺), phosphate (PO₄ ³⁻), nitrate (NO₃ ⁻) and nitrite (NO₂ ⁻) concentrations in the aqueous phase. A maximum energy recovery of 78% was obtained at 350 °C and 5 min, with a high energy density of 34.3 MJ/kg and lower contents of oxygen. For the recovery of inorganic nutrients, shorter reaction times achieved higher phosphorus recovery, with maximum recovery being 53% at 350 °C and 5 min. Our results indicate that the reaction condition of 350 °C for 5 min was optimal for maximizing energy recovery with improved quality, at the same time achieving a high phosphorus recovery.